The morning of September 11, 2001, started off like any other. The Twin Towers stood tall in the Financial District, as they had for more than 30 years.

Fernando Llano / AP

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At 8:46 a.m., American Flight 11 crashed into the North Tower of the World Trade Center. At first, newscasters weren't sure whether it was an accident or a deliberate attack.

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At 9:03 a.m., United Flight 175 crashed into the South Tower, leaving no doubt that this was an attack. Some news channels captured the moment on live television.

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The second plane exploded upon impact, caused by the igniting of fuel. Now both buildings were burning.

AP Photo/Chao Soi Cheong

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People stared from the windows of the Towers, trapped by smoke and flames and destroyed staircases.

REUTERS/Jeff Christensen

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This iconic photo captured a man falling from the North Tower. At least 200 people fell or jumped from the Towers.

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At 9:40 a.m., American Flight 77 crashed into the Pentagon. Five minutes later, for the first time in history, the FAA ordered all aircraft to land at the nearest airport.

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At 9:59 a.m. the South Tower collapsed.

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People fleeing the scene by foot were covered in dust and ash.

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At 10:03 a.m., hijacked flight United Flight 93 crashed into a field in Shanksville, Pennsylvania. The plane's target was believed to be the US Capitol. The passengers on board tried to gain control of the flight and divert the hijackers after learning of the other attacks.

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At 10:28 a.m. the North Tower collapsed. It took only 12 seconds for the Towers to fall.

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2,753 people were killed in the New York attack.

An aerial view of the World Trade Center disaster site seen on September 18, 2001.
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That number includes 343 firefighters and paramedics and 60 police officers who rushed to help in the aftermath.

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Another 40 people were killed in Pennsylvania and 184 died in Washington, D.C., for a total of nearly 3,000 people.

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Rescue efforts at Ground Zero continued until October 9, and the flames from the collapse burned until December 20.

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In the months after 9/11, the nation came together to help those affected by the attacks. Blood banks were overwhelmed with donations, and hundreds of people volunteered to sift through rubble at Ground Zero.

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After the terrorist attacks, President Bush declared a "War on Terror," targeting the Al Qaeda terrorists responsible for the attack. Al Qaeda's leader, Osama Bin Laden, was killed 10 years later. The national response included a large expansion of America's security efforts.

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This is the skyline today, with the Tribute in Light marking the spot where the towers once were.

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Now, the 9/11 Memorial is open to the public to commemorate the tragedy.